Allow easy override of params in components that extend core-ajax.

core-ajax.html

@@ -286,6 +286,14 @@
       }
     },
 
+    getParams: function(params) {
+      params = this.params || params;
+      if (params && typeof(params) == 'string') {
+        params = JSON.parse(params);
+      }
+      return params;
+    },
+
     /**
      * Performs an Ajax request to the specified URL.
      *
@@ -295,10 +303,7 @@
       var args = this.xhrArgs || {};
       // TODO(sjmiles): we may want XHR to default to POST if body is set
       args.body = this.body || args.body;
-      args.params = this.params || args.params;
-      if (args.params && typeof(args.params) == 'string') {
-        args.params = JSON.parse(args.params);
-      }
+      args.params = this.getParams(args.params);
       args.headers = this.headers || args.headers || {};
       if (args.headers && typeof(args.headers) == 'string') {
         args.headers = JSON.parse(args.headers);